The invention relates to a process of preparing a multilayer cholesteric film. The invention further relates to a multilayer film obtainable by the inventive process, and to the use of the inventive process and a film prepared thereby in optical elements such as polarizers, compensators, alignment layers, color filters or holographic elements, in cosmetics and liquid crystal pigments, for decorative and security applications.
Cholesteric liquid crystal (CLC) materials, when formed into thin layers with planar alignment, i.e. wherein the cholesteric helix axis is oriented substantially perpendicular to the plane of the layer, exhibit the well-known effect of selective reflection of light, where the wavelength of the reflected light is dependent on the pitch of the cholesteric helix. By using polymerizable CLC materials, the aligned CLC layer can be converted to a coherent polymer film that retains the selective reflection properties of the original material.
CLC polymer films are known in prior art and have been proposed for a variety of uses, for example as broadband or notch polarizers, as color filters in displays or projection systems, and for decorative and security purposes, like the preparation of colored image films or cholesteric pigment flakes.
For some applications, it is desirable to form a multilayer cholesteric film, comprising two or more cholesteric layers e.g. exhibiting different reflection wavelength.
Multilayer cholesteric polymer films have been described in prior art. For example, EP 0 634 674 suggests to prepare a multilayer cholesteric liquid crystal polymer film by bringing together a pair of chiral nematic liquid crystal polymer films, applying pressure and heating the polymers above their glass transition temperature to allow the films to adhere.
Maurer et al., SID 90 Digest, Vol .21, pp. 110 (1990) describes a polarizing color filter obtained by combining several polarizing films with different reflection wavelength. For the preparation of each film, a layer of a CLC side chain polysiloxane comprising chiral and achiral sidegroups is brought between two glass plates and oriented by shearing at high temperatures.
JP 01-133003-A (Sumitomo Chem. Ind.) and JP 08-271731-A (Nitto Denko) disclose polarizing plates that are obtained by lamination of one or more CLC polymer layers onto a quarter wave plate.
However, the methods of preparing multilayer cholesteric films as described in the above documents bear several disadvantages. Thus, it is often very difficult and requires high temperatures to achieve uniform alignment in the CLC polymer layer. For example, Maurer et al. mentions an aligning temperature of 150xc2x0 C., whereas JP 01-133003-A and JP 08-271731-A mention that temperatures well above the glass temperature of the CLC polymers are required. This is especially disadvantageous when polymers with high glass temperatures, like acrylates, styrenes or methacrylates are used, and is highly unsuitable in particular for mass production.
Furthermore, according to the method of multilayer preparation as described e.g. in JP 01-133003-A, the polymers have to be selected such that the different polymer layers exhibit different glass temperatures. Thus, when laminating and aligning ,e.g., a second layer on top of a first layer, the aligning temperature (and thus the glass temperature) of the second layer has to be lower than the glass temperature of the first layer, so as not to affect the uniform orientation of the first layer, etc. This severely limits the choice of suitable materials and makes the production process more complicated.
Therefore, there is a need for a method to prepare a multilayer cholesteric film in an efficient and cost-effective manner that does not have the above mentioned drawbacks, allows better control of the reflection wavelength and is suitable for mass production.
One aim of the present invention is to provide a process of preparing a multilayer cholesteric film that fulfills the above requirements. Other aims of the invention are immediately evident to a person skilled in the art from the following description.
A simple way to prepare a multilayer cholesteric film is by first coating and polymerizing a cholesteric layer of one wavelength, and then onto this directly coating and polymerizing a second cholesteric layer, e.g. of a different wavelength.
In this case it is important to ensure that the helical pitch, and thus the reflection wavelength of either layer, does not change in an uncontrollable manner.
In some cases, however, it is advantageous if the variation of the helical pitch can be controlled.
The inventors of the present invention have found that by controlled migration of unpolymerized material between adjacent layers, in particular migration of unpolymerized chiral material from the first, polymerized layer to the second, still unpolymerized layer, it is possible to alter the helical pitch of the second layer, whilst the pitch of the first layer remains substantially unchanged, depending on the extent of polymerization and the crosslinking density of the first layer.
Furthermore, the inventors found that, when preparing a multilayer cholesteric film, it is possible to control the migration of unpolymerized chiral material, and thus to alter the helical pitch in the single cholesteric layers prior to their polymerization, by using a process according to the present invention, which will be described in detail below.
One object of the present invention is a process of preparing a multilayer cholesteric film comprising two or more layers of polymerized cholesteric liquid crystal (CLC) material with planar orientation, wherein
the first layer is prepared by applying a layer of polymerizable CLC material on a substrate, preferably by coating, optionally aligning the CLC material in a planar orientation, and polymerizing the aligned CLC material, for example by means of heat or actinic radiation, and
the second and higher layers are prepared by applying a layer of polymerizable CLC material on a substrate, preferably by coating, optionally aligning the CLC material in a planar orientation, and polymerizing the aligned CLC material, for example by means of heat or actinic radiation, wherein one of the previously polymerized layers serves as substrate,
characterized in that the cholesteric helical pitch of adjacent layers is varied by controlled migration of a non-polymerized chiral material between the layers.
Another object of the present invention is a multilayer cholesteric film obtainable by a process as described above and below.
Another object of the present invention relates to the use of a process as described above and below, and of a multilayer cholesteric film obtainable by this process, in optical elements such as polarizers, compensators, alignment layers, color filters or holographic elements, in cosmetics and liquid crystal pigments, for decorative and security applications.
FIG. 1 depicts the visible light absorption spectrum of two-layer cholesteric films prepared from two polymerizable CLC mixtures with different reflection wavelength in varying order, compared to the spectrum of the one-layer films prepared from the CLC mixtures.
FIG. 2 depicts the visible light absorption spectrum of two-layer cholesteric films prepared from two polymerizable CLC mixtures with different reflection wavelength, wherein the second layer was annealed for various time periods before being polymerized, compared to the spectrum of the one-layer films prepared from the CLC mixtures.
The term xe2x80x98layer of cholesteric liquid crystal material with planar orientationxe2x80x99 means a liquid crystal material with a cholesteric phase, i.e. wherein the mesogens are oriented with their main molecular axis in a preferred direction within molecular sublayers, said preferred orientation direction in different sublayers being twisted around a helix axis that is substantially perpendicular to the plane of the layer, i.e. substantially parallel to the layer normal. This definition also includes orientations where the helix axis is tilted at an angle of up to 2xc2x0 relative to the layer normal.
Especially preferred are multilayer cholesteric films wherein adjacent layers have different helical pitch.
Further preferred are multilayer cholesteric films wherein the polymerized CLC material, in particular in the first layer, forms a three-dimensional polymer network.
The polymerizable CLC material according to the present invention is a mixture comprising one or more polymerizable achiral mesogenic compounds, and further comprising one or more polymerizable chiral mesogenic compounds and/or one or more non-polymerizable chiral dopants that induce the cholesteric liquid crystalline phase behavior in the mixture.
A first preferred embodiment of the present invention relates to a process described herein, wherein the polymerizable CLC material of the first layer contains a defined amount of a non-polymerizable chiral dopant.
When a second layer of polymerizable CLC material is coated on top of the first, polymerized layer comprising the non-polymerizable chiral dopant, the dopant migrates from the first into the second layer, before the second layer is polymerized. Migration of the dopant causes a shift in the cholesteric pitch of the second layer to shorter wavelengths.
The rate of migration of the dopant, and therefore the ultimate pitch length of the second layer, is controlled by the temperature, the annealing time and temperature and, if the CLC material of the second layer is dissolved or dispersed in a solvent, by the type of the solvent used for the coating process.
The second layer is then also polymerized to fix the cholesteric structure.
It is preferred that at least the polymerized material in the first layer forms a three-dimensional network. It is in particular preferred that the first layer has a degree of crosslinking that is high enough so that its helical pitch does not change, or change only minimally, as the chiral dopant migrates out of the layer. At the same time, however, the degree of crosslinking should not be too high, in order not to enhance migration of chiral dopants out of the layer, e.g. due to macroscopic phase separation.
Further preferably the CLC material of the second and higher layers also form a three-dimensional network upon polymerization, so that their helical pitch does not change, or change only minimally, after polymerization of this layer when the next layer is coated on top of it.
In particular chiral unpolymerized compounds migrating between the layers do alter the helical pitch (and therefore the reflection wavelength) of the unpolymerized layer. The change of the reflection wavelength increases both with increasing amount and increasing helical twisting power (HTP) of the chiral material.
The helical twisting power (HTP) of a chiral dopant inducing a cholesteric phase in a nematic liquid crystal host mixture is defined by equation (1):                     p        =                              1            HTP                    ·                      1            c                                              (        1        )            
wherein p is the pitch of the cholesteric helix and c is the concentration of the chiral dopant.
The reflection wavelength xcex of a CLC material comprising the nematic host and chiral dopant of above equation (1) is given in first approximation by equation (2):
xcex=nxc2x7pxe2x80x83xe2x80x83(2)
wherein p is the pitch and n the mean refractive index of the cholesteric material.
Combining equations (1) and (2) leads to:
xcex=nxc2x7HTPxe2x88x921xc2x7cxe2x88x921xe2x80x83xe2x80x83(3)
The migration of unpolymerized material between two adjacent layers of CLC material changes the concentration c of the chiral material in the layer and thus leads to a change of the reflection wavelength as long as the CLC material is not polymerized.
However, in case of a polymerized CLC layer, for example a loss of unpolymerized chiral material will not lead to a significant change of the reflection wavelength, since the twisted cholesteric structure of the layer is fixed by polymerization as described above.
According to this invention, the relative change of the reflection wavelength in a second layer of polymerizable CLC material coated onto a first layer of polymerized CLC material due to migration of unpolymerized material into or out of the second layer is defined as |xcexxe2x88x92xcex0|/xcex0, wherein
xcex0 is the wavelength of the center of the reflection peak of the CLC material of said second layer when polymerized on a polyimide-coated glass substrate, and
xcex is the wavelength of the center of the reflection peak of the CLC material of said second layer when polymerized on said first layer.
The substrate used for preparing the second layer as a single layer to determine xcex0 was shown to have no significant influence on the reflection wavelength of the polymerized CLC material. As standard substrate a glass substrate (sodalime without ITO from Microtech Corp., Japan) coated with polyimide (AL-1054 from JSR Ltd. Co, Japan, applied by spin-coating and rubbed unidirectionally) was used.
The first and second layer can be made of identical or different polymerizable CLC materials.
Preferably, in a process according to the present invention migration of chiral material between two adjacent layers is minimized such that |xcexxe2x88x92xcex0|/xcex0xe2x89xa60.2, in particular xe2x89xa60.1, very preferably xe2x89xa60.05, most preferably xe2x89xa60.02.
In case of a polymerizable CLC material comprising both chiral polymerizable mesogenic compounds and non-polymerizable chiral dopants, it is possible that both unreacted chiral polymerizable mesogenic compounds and non-polymerizable dopant migrate from the first, polymerized layer into the second, unpolymerized layer.
The more completely the first layer is polymerized, the lower will be the amount of material that is polymerizable, but unreacted after polymerization, which can migrate into the second layer. At the same time, this makes the first, polymerized layer more impenetrable to the material, e.g. the solvent, from the second, still unpolymerized layer, as the first polymerized layer is more coherent.
Therefore, in a preferred embodiment of the present invention relates to a process according to claim 1, wherein the extent of polymerization of crosslinking of the first layer is maximized, so that migration of unreacted polymerizable material is minimized. In this way, the migration process be better controlled by appropriate selection of type and amounts of non-polymerizable material, whereas uncontrolled migration of unreacted polymerizable material is reduced.
The extent of polymerization is preferably xe2x89xa760%, in particular xe2x89xa780 %, most preferably xe2x89xa795%.
The minimum degree of crosslinking is typically 5-10%. Preferably the degree of crosslinking is 10-40%, most preferably 40-60%.
In another preferred embodiment of the present invention, the polymerizable CLC material is comprising chiral polymerizable mesogenic compounds with a low or moderate HTP and non-polymerizable chiral dopants with a high HTP. In this case, the effect of the unpolymerized dopant on the wavelength of the second layer is significantly stronger compared to the effect of the unreacted polymerizable material, whereas the latter effect is much smaller.
Thus, the effect of non-polymerizable chiral dopants migrating from the first into the second layer and changing the reflection wavelength of the second layer can be controlled by selection of the amount and type of the chiral dopant, the latter being chosen e.g. according to its HTP or migration rate.
In case of high HTP dopants even the migration of small amounts into the unpolymerized cholesteric layer can lead to a considerable change of its reflection wavelength. Therefore, according to the inventive method both the amount and the HTP of the dopant have to be balanced against each other.
Preferably the polymerizable CLC material of the first layer comprises 0.1 to 15%, in particular 0.2 to 10%, very preferably 0.5 to 5% by weight of non-polymerizable chiral dopants.
Particularly preferred are dopants with a HTP  greater than 20 xcexcmxe2x88x921, very preferably with a HTP  greater than 50 xcexcmxe2x88x921.
Further to non-polymerizable chiral dopants the polymerizable CLC material may also contain other components not carrying a polymerizable group, like photoinitiators, dyes, surfactants, inhibitors and other additives. The total amount of these non-polymerizable components is preferably less than 5%, in particular less than 3%, very preferably less than 1% by weight of the total mixture.
A second preferred embodiment of the present invention relates to a process, wherein the migration is controlled by using the polymerizable CLC material of at least one of the second and higher layers in combination with a solvent that is selected to have a defined penetrating effect on the previously polymerized layer.
Thus, the CLC material is dissolved or dispersed in a solvent for the coating process. Penetration of the solvent into the first, polymerized layer greatly enhances the migration of material from the first layer into the second layer. This can lead to a change in the helical pitch of the second layer. On the other hand, if the polymerized material in the first layer becomes swollen by the solvent, or by low molecular weight components from the second layer, the resulting expansion of the layer causes the pitch length to increase with a shift in the reflection band to longer wavelengths.
The migration of material into the second layer can therefore be controlled by selecting a solvent having a defined penetrating effect on the first, polymerized layer. The penetrating effect of the solvent can be determined by measuring the degree of swelling of the polymerized material of the first layer in the solvent.
Preferably a solvent is used wherein the polymerized CLC material of the previously polymerized layer has a high degree of swelling.
Another way to determine the penetrating effect of a solvent is to measure the change of reflection wavelength of an inventive polymerized layer of CLC material before and after treatment with the solvent. This is exemplarily illustrated in example 3.
A solvent with high penetrating power is effective at removing non-polymerized material from the polymer film which then shrinks and its reflection wavelength becomes shorter. Thus, if the change of reflection wavelength after solvent treatment is small, the solvent has a small penetrating effect.
Particularly preferred are solvents causing a change of reflection wavelength |xcex94xcex/xcex|xe2x89xa70.05, in particular xe2x89xa70.1, very preferably xe2x89xa70.2, wherein xcex94xcex= (xcexbefore-xcexafter) is the difference between the central reflection wavelength of a polymerized CLC layer before solvent treatement (xcexbefore) and after solvent treatment (xcexafter).
Preferred solvents are toluene, acetone, tetrahydrofuran, ethyl acetate and butanone, for example.
A third preferred embodiment of the present invention relates to a process, wherein the migration is controlled by annealing the polymerizable CLC material of at least one of the second and higher layers is annealed for a defined time and at a defined temperature below the cholesteric-isotropic phase transition temperature of the CLC material prior to polymerization.
Thus, it may be necessary for the second layer to be annealed at an elevated temperature prior to curing in order to get good alignment. In this case the migration of material (particularly chiral dopant) between layers can be controlled by varying the annealing temperature and time. Thus, migration is promoted by increases in both temperature and time of the annealing process (see example 2). If the annealing time is sufficiently long, there can be a significant change in the reflection wavelength of the second layer.
The annealing temperature is preferably 1 to 40xc2x0 C., in particular 1 to 30xc2x0 C., very preferably 1 to 10xc2x0 C. below the cholesteric-isotropic phase transition temperature of the polymerizable CLC material.
The annealing time is preferably up to 5 minutes, in particular up to 3 minutes, very preferably from 10 to 60 seconds, For specific applications short annealing times of 1 to 30 seconds, in particular 1 to than 10 seconds, very preferably up to 5 seconds, are preferred.
A fourth preferred embodiment of the present invention relates to a process according to claim 1, wherein adjacent layers are prepared in a sequence where the pitch of the CLC material of the layer that is prepared first is shorter than the pitch of the CLC material of the layer that is prepared next.
The exact difference in pitch between adjacent layers depends on the final application that the cholesteric film is intended to be used for. Typically the difference is in the range of 30-300 nm.
In a fifth preferred embodiment, the first and second layers contain chiral dopants of opposite handedness of the twist. For example, the first layer contains a dopant inducing a twisted structure with right-handed twist sense, and the second layer contains a dopant inducing a twisted structure with left-handed twist sense. Migration of the dopant from the first layer then causes a shift of the reflection wavelength in the second layer to longer wavelengths, and the adjacent layers then reflect circularly polarized light of defined wavelengths and opposite twist.
A sixth preferred embodiment of the present invention relates to a method of patterning a cholesteric layer.
For example, a first cholesteric layer is coated and polymerized, having a pitch length e.g. such that its reflection is in the UV region. A second layer with a pitch in the red or IR region is coated on top of the first layer. The second layer is then annealed for a given period of time at a given temperature until its pitch length has moved to a new, shorter value. The second layer is then irradiated through a mask to polymerize the material and thereby fix the cholesteric helix, but only in the defined areas that are not covered by the mask and thus exposed to radiation.
The annealing process is then repeated to cause a further shift in the pitch length in those areas that have not been polymerized. Further irradiation through a different mask then fixes these previously unexposed areas of the film with a different helical pitch.
This process can be repeated as many times as is necessary to produce a range of different pitch lengths distributed over the area of the sample. A further elaboration would be to coat a third, fourth or higher number of layers and to repeat this process.
Patterned cholesteric films are particularly suitable as color filters and for decorative and securtiy applications.
A seventh preferred embodiment of the present invention relates to a method of preparing a broadband cholesteric film reflecting a continuous, broad band of wavelengths. Preferably such a film relfects a waveband covering a substantial portion of the visible range of light, in the ideal case covering the entire visible spectrum.
Such a broadband film is obtained by a migration process as described above, by means of which a gradient in the chiral dopant concentration, and therefore in the pitch length, is created through the thickness of the multilayer film.
Such a broadband film would be useful in particular as broadband circular polarizer. Broadband circular polarizers are described e.g. in EP 0 606 940, WO 97/35219 and WO 97/16762.
Diffusion in the polymerizable CLC material as used in the above described preferred embodiments, in particular when used with a solvent, is typically rapid due to the low viscosity of the material.
Therefore, the CLC material according to the seventh preferred embodiment should preferably exhibit an increased viscosity. This allows to slow down, and thus better control, the diffusion within one layer. This can be achieved by addition of an oligomer or polymer to the CLC material preferably a liquid crystal side chain polymer to ensure good compatibility with the polymerizable mixture. Suitable oligomers and polymers are for example oligo- or polyacrylates, -methacrylates, -ethylenes or -epoxides with mesogenic side chains as shown in formula I-V and their preferred subformulae below. The amount of added oligomer or polymer is preferably xe2x89xa625% by weight of the total material.
It is also possible to optimize the annealing temperature to achieve the right diffusion rate, and/or to optimize the layer thickness. Thus, higher layer thickness, longer annealing times and/or a lower annealing temperatures are preferred, as they enable better control of the diffusion process.
The thickness of the single layers of the inventive multilayer film is preferably less than 5 xcexcm, in particular from 1 to 5 xcexcm, very preferably from 3 to 5 xcexcm.
In a further elaboration of the present invention, the first layer does itself not necessarily have to be a cholesteric layer. It is also possible that the first layer is an isotropic polymer layer. Thus, it is possible to apply e.g. a layer of a polymerizable or crosslinkable non-LC polymer precursor comprising e.g. polymerizable or crosslinkable non-LC monomers or oligomers onto a substrate, wherein said non-LC polymer precursor comprises a specific amount of one or more chiral dopants that are unreactive or do not completely react under the conditions of the polymerization of the first layer. The first layer is then polymerized or crosslinked. A second layer of a polymerizable CLC material is then coated onto the first layer, aligned and polymerized, wherein unreacted or non-polymerizable chiral compounds of the first, isotropic layer migrate into the second, cholesteric layer prior to its polymerization, thereby altering the pitch in said second CLC layer. The second layer of polymerizable CLC material may also be sandwiched between two isotropic layers.
Alternatively it is also possible to use as first layer a layer of an isotropic polymer comprising one or more unpolymerized chiral compounds, said polymer then optionally being crosslinked. The chiral component can be dispersed in the polymer before polymerization, or by swelling the polymer in a solution of the chiral component e.g. in an organic solvent.
In this case, the first polymerized layer merely serves as a reservoir providing the chiral compounds that migrate into the second or higher layers. The methods to control the migration rate of the chiral compounds as described for the preferred embodiments above and below can be applied to the isotropic polymer layer analoguously. This embodiment is especially suitable, and preferably used, for the sixth and seventh preferred embodiment described above.
Suitable polymerizable isotropic, i.e. non-mesogenic materials are for example commercially available monomers like acrylates, methacrylates or epoxides, like ethylhexyl acrylate, hexanediol acrylate, trimethylolpropane triacrylate, and acrylated oligomers.
Examples for difunctional non mesogenic monomers are alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1 to 20xc2x0 C. atoms. Typical examples for non mesogenic monomers with more than two polymerizable groups are trimethylpropanetrimethacrylate or pentaerythritoltetraacrylate.
In a preferred embodiment the polymerizable CLC mixture comprises at least one polymerizable mesogenic compound having two or more polymerizable functional groups (di- or multireactive or di- or multifunctional compound). Upon polymerization of such a mixture a three-dimensional polymer network is formed. A polymer film made of such a network is self-supporting and shows a high mechanical and thermal stability and a low temperature dependence of its liquid crystal properties. In the ideal case the liquid crystal properties of the polymer film are temperature independent.
By varying the concentration of the multifunctional mesogenic or non mesogenic compounds, the crosslink density of the resulting polymer film and thereby its physical and chemical properties such as the glass transition temperature, which is also important for the temperature dependence of the optical properties of the film, the thermal and mechanical stability or the solvent resistance can be tuned easily.
Especially preferably the polymerizable CLC mixture comprises 5 to 100%, in particular 25 to 80%, very preferably 45 to 70%, by weight of multireactive mesogenic compounds. Direactive mesogenic compounds are particularly preferred.
The polymerizable components of the CLC material according to the present invention are preferably selected of formula I
Pxe2x80x94(Spxe2x80x94X)nxe2x80x94MGxe2x80x94Rxe2x80x83xe2x80x83I
wherein
P is CH2xe2x95x90CWxe2x80x94COOxe2x80x94, WCHxe2x95x90CHxe2x80x94Oxe2x80x94, 
xe2x80x83or CH2xe2x95x90CH-Phenylxe2x80x94(O)kxe2x80x94 with W being H, CH3 or Cl and k being 0 or 1,
Sp is a spacer group having 1 to 25 C atoms,
X is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94,xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94COxe2x80x94NHxe2x80x94, xe2x80x94NHxe2x80x94COxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94SCH2xe2x80x94, xe2x80x94CH2Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, or a single bond,
n is 0 or 1,
MG is a mesogenic group, and
R is H, CN, halogen or a straight-chain or branched alkyl radical with up to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH2 groups to be replaced, in each case independently from one another, by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94N(CH3)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOxe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 in such a manner that oxygen atoms are not linked directly to one another, or alternatively R is denoting Pxe2x80x94(Spxe2x80x94X)nxe2x80x94.
MG in formula I is preferably selected of formula II
xe2x80x94A1xe2x80x94Z1xe2x80x94(A2xe2x80x94Z2)mxe2x80x94A3xe2x80x94xe2x80x83xe2x80x83II
wherein
Z1 and Z2 are each independently xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x95x90Cxe2x80x94, or a single bond,
A1, A2 and A3 are each independently 1,4-phenylene in which, in addition, one or more CH groups may be replaced by N, 1,4-cyclohexylene in which, in addition, one or two non-adjacent CH2 groups may be replaced by 0 and/or S, 1,4-cyclohexenylene, 1,4-bicyclo(2,2,2)octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl, or 1,2,3,4-tetrahydro-naphthalene-2,6-diyl, it being possible for all these groups to be unsubstituted, mono- or polysubstituted with F,CI, OH, CN or NO2 or alkyl, alkoxy or alkanoyl groups having 1 to 7 C atoms wherein one or more H atoms may be substituted by F or Cl, and
m is 0, 1 or 2.
A smaller group of preferred mesogenic groups of formula II is listed below. For reasons of simplicity, Phe in these groups is 1,4-phenylene, PheL is a 1,4-phenylene group which is substituted by 1 to 4 groups L, with L being F, Cl, OH, CN, NO2 or an optionally fluorinated alkyl, alkoxy or alkanoyl group with 1 to 7 C atoms, and Cyc is 1,4-cyclohexylene. The following list of preferred mesogenic groups is comprising the subformulae II-1 to II-24 as well as their mirror images
Particularly preferred are the subformulae II-1, II-2, II-4, II-6, II-7, II-8, II-11, II-13, II-14, II-15 and II-16.
In these preferred groups Z in each case independently has one of the meanings of Z1 as given in formula I. Preferably Z is xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond.
Very preferably the mesogenic group MG is selected from the following formulae and their mirror images 
wherein L has the meaning given above and r is 0, 1 or 2.
The group 
in these preferred formulae is very preferably denoting 
or 
Furthermore 
with L having each independently one of the meanings given above. Particularly preferred are the subformulae IId, IIg, IIh, IIi, IIk and IIo, in particular the subformulae IId and IIk.
L is preferably F, Cl, CN, OH, NO2, CH3, C2H5, OCH3, OC2H5, COCH3, COC2H5, COOCH3, COOC2H5, CF3, OCF3, OCHF2, OC2F5, in particular F, Cl, CN, CH3, C2H5, OCH3, COCH3 and OCF3, most preferably F, Cl, CH3, OCH3 and COCH3.
In case of a polymerizable mesogenic compounds with a terminal polar group, R is preferably alkyl with up to 15 C atoms or alkoxy with 2 to 15 C atoms.
If R in formula I is an alkyl or alkoxy radical, i.e. where the terminal CH2 group is replaced by xe2x80x94Oxe2x80x94, this may be straight-chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8 carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, or octoxy, furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy, for example.
Oxaalkyl, i.e. where one CH2 group is replaced by xe2x80x94Oxe2x80x94, is preferably straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3-, or 4-oxapentyl, 2-, 3-, 4-, or 5- oxahexyl, 2-, 3-, 4-, 5-, or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-,7-, 8- or 9-oxadecyl, for example.
In case of a polymerizable mesogenic compounds with a terminal polar group, R is selected from CN, NO2, halogen, OCH3, OCN, SCN, COR1, COOR1 or a mono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 C atoms. R1 is optionally fluorinated alkyl with 1 to 4, preferably 1 to 3 C atoms. Halogen is preferably F or Cl. Especially preferably R in these compounds is selected of F, Cl, CN, NO2, OCH3, COCH3, COC2H5, COOCH3, COOC2H5, CF3, C2F5, OCF3, OCHF2, and OC2F5, in particular of F, Cl, CN, OCH3 and OCF3.
In the compounds of formula I R may be an achiral or a chiral group. In case of a chiral group it is preferably selected according to the following formula III: 
wherein
X1 is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOOxe2x80x94 or a single bond,
Q1 is an alkylene or alkylene-oxy group with 1 to 10 C atoms or a single bond,
Q2 is an alkyl or alkoxy group with 1 to 10 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH2 groups to be replaced, in each case independently from one another, by xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94N(CH3)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOxe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94 or xe2x80x94COxe2x80x94Sxe2x80x94 in such a manner that oxygen atoms are not linked directly to one another,
Q3 is halogen, a cyano group or an alkyl or alkoxy group with 1 to 4 C atoms different from Q2.
In case Q1 in formula IlIl is an alkylene-oxy group, the O atom is preferably adjacent to the chiral C atom.
Preferred chiral groups R are 2-butyl (=1-methylpropyl), 2-methylbutyl, 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, 2-octyl, in particular 2-methylbutyl, 2-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, 2-octyloxy, 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl, 6-methoxyoctoxy, 6-methyloctoxy, 6-methyloctanoyloxy, 5-methylheptyloxycarbonyl, 2-methylbutyryloxy, 3-methylvaleroyloxy, 4-methylhexanoyloxy, 2-chlorpropionyloxy, 2-chloro-3-methylbutyryloxy, 2-chloro-4-methylvaleryloxy, 2-chloro-3-methylvaleryloxy, 2-methyl-3-oxapentyl, 2-methyl-3-oxahexyl, 1-methoxypropyl-2-oxy, 1-ethoxypropyl-2-oxy, 1-propoxypropyl-2-oxy, 1 -butoxypropyl-2-oxy, 2-fluorooctyloxy, 2-fluorodecyloxy for example.
In addition, compounds of formula I containing an achiral branched group R may occasionally be of importance, for example, due to a reduction in the tendency towards crystallization. Branched groups of this type generally do not contain more than one chain branch. Preferred achiral branched groups are isopropyl, isobutyl (=methylpropyl), isopentyl (=3-methylbutyl), isopropoxy, 2-methyl-propoxy and 3-methylbutoxy.
Another preferred embodiment of the present invention relates to compounds of formula I wherein R is denoting Pxe2x80x94(Spxe2x80x94X)nxe2x80x94.
P in formula I is preferably an acrylate group, a methacrylate group, a vinyl or vinyloxy group, an epoxy group, a styrene group or a propenyl ether group, in particular an acrylate, methacrylate, vinyl or epoxy group.
As for the spacer group Sp in formula I all groups can be used that are known for this purpose to the skilled in the art. The spacer group Sp is preferably a linear or branched alkylene group having 1 to 20 C atoms, in particular 1 to 12 C atoms, in which, in addition, one or more non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94N(CH3)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COOxe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94CH(halogen)xe2x80x94, xe2x80x94CH(CN)xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94.
Typical spacer groups are for example xe2x80x94(CH2)oxe2x80x94, xe2x80x94(CH2CH2O)pxe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94Sxe2x80x94CH2CH2xe2x80x94 or xe2x80x94CH2CH2xe2x80x94NHxe2x80x94CH2CH2xe2x80x94, with o being an integer from 2 to 12 and p being an integer from 1 to 3.
Preferred spacer groups are ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, octadecylene, ethyleneoxyethylene, methyleneoxybutylene, ethylene-thioethylene, ethylene-N-methyl-iminoethylene, 1-methylalkylene, ethenylene, propenylene and butenylene for example.
Especially preferred are inventive compounds of formula I wherein Sp is denoting an alkyl or alkoxy group with 2 to 8 C atoms. Straight-chain alkyl or alkoxy groups are especially preferred.
In another preferred embodiment of the invention the chiral compounds of formula I comprise at least one spacer group Sp that is a chiral group of the formula IV: 
wherein
Q1 and Q3 have the meanings given in formula III, and
Q4 is an alkylene or alkylene-oxy group with I to 10 C atoms or a single bond, being different from Q1.
In case Q1 in formula III is an alkylene-oxy group, the O atom is preferably adjacent to the chiral C atom.
In the event that R is denoting Pxe2x80x94Spxe2x80x94Xxe2x80x94, the two spacer groups Sp in the compounds of formula I may be identical or different.
Of the preferred compounds described above particularly preferred are those wherein n is 1.
Further preferred are compounds comprising both a group Pxe2x80x94(Spxe2x80x94X)nxe2x80x94wherein n is 0 and a group Pxe2x80x94(Spxe2x80x94X)nxe2x80x94 wherein n is 1.
The compounds of formula I can be synthesized according to or in analogy to methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart. Some specific methods of preparation can be taken from the examples.
Examples of suitable polymerizable mesogenic compounds that can be used as components of the polymerizable CLC material, are disclosed for example in WO 93/22397; EP 0,261,712; DE 195,04,224; WO 95/22586 and WO 97/00600. The compounds disclosed in these documents, however, are to be regarded merely as examples that shall not limit the scope of this invention. Preferably the polymerizable CLC mixture comprises at least one polymerizable mesogenic compound having one polymerizable functional group and at least one polymerizable mesogenic compound having two or more polymerizable functional groups.
Examples of especially useful monoreactive chiral and achiral polymerizable mesogenic compounds are shown in the following list of compounds, which should, however, be taken only as illustrative and is in no way intended to restrict, but instead to explain the present invention: 
wherein, P has one of the meanings of formula I and its preferred meanings as mentioned above, x is an integer from 1 to 12 , A is 1,4-phenylene or 1,4-cyclohexylene, v is 0 or 1, Y is a polar group as defined above, R0 is an unpolar alkyl or alkoxy group, Ter is a terpenoid radical like e.g. menthyl, Chol is a cholesteryl group, and L1 and L2 are each independently H, F, Cl, CN, OH, NO2 or an optionally halogenated alkyl, alkoxy or carbonyl group with 1 to 7 C atoms.
The term unpolar group in this connection preferably denotes an alkyl group with 1 or more, preferably 1 to 15 C atoms or an alkoxy group with 2 or more, preferably 2 to 15 C atoms.
Examples of useful direactive chiral and achiral polymerizable mesogenic compounds are shown in the following list of compounds, which should, however, be taken only as illustrative and is in no way intended to restrict, but instead to explain the present invention 
wherein P, x, D, L1 and L2 have one of the meanings given above and y is an integer from 1 to 12 the same as or different from x.
A polymerizable CLC material according to the first preferred embodiment as described above comprises one or more chiral dopants which themselves do not necessarily have to show a liquid crystalline phase and give good planar alignment themselves, in particular non-polymerizable chiral dopants.
Especially preferred are chiral dopants with a high helical twisting power (HTP), in particular those disclosed in WO 98/00428. Further typically used chiral dopants are e.g. the commercially available S 1011, R 811 or CB 15 (from Merck KGaA, Darmstadt, Germany).
Especially preferred are chiral non-polymerizable dopants selected from the following formulae 
with formula VIII including the (S,S) enantiomer, wherein E and F are each independently 1,4-phenylene or trans-1,4-cyclohexylene, v is 0 or 1, Z0 is xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond, and R is alkyl, alkoxy or alkanoyl with 1 to 12 C atoms.
The compounds of formula VII and their synthesis are described in WO 98/00428. The compounds of formula VIII and their synthesis are described in GB 2,328,207.
The above chiral compounds of formula VII and VII exhibit a very high helical twisting power (HTP), and are therefore particularly useful for the purpose of the present invention.
A polymerizable CLC material according to the second preferred embodiment as described above comprises one or more chiral mesogenic compounds that should exhibit planar alignment themselves, in particular polymerizable chiral compounds of this type. Preferably these compounds are selected of formula 1, wherein Sp and/or R are comprising a chiral moiety. Particularly preferred are chiral compounds selected of the formula Va to Vm above.
A polymerizable CLC material according to the fourth preferred embodiment as described above comprises a surface active component comprising one or more surfactants.
Suitable surfactants are described for example in J. Cognard, Mol.Cryst.Liq.Cryst. 78, Supplement 1, 1-77 (1981). Particularly preferred are non-ionic surfactants, such as mixtures of non-ionic fluoroalkylalkoxylate surfactants selected from the following formulae
wherein n is an integer from 4 to 12 and x is an integer from 5 to 15. Upon using these surfactants it is possible to produce polymerized films with a low tilt angle in the range from 0 to 1 degrees, in particular 0 to 0.5 degrees. In the best case the tilt angle is approximately 0 degrees.
The surfactants of formula IX are commercially available under the trade name Fluorad 171 (from 3M Co.), the surfactants of formula X under the trade name Zonyl FSN (from DuPont).
In case surfactants are used, the CLC material contains preferably from 500 to 2500 ppm, in particular from 1000 to 2500 ppm, very preferably from 1500 to 2500 ppm of surfactants. Further preferred materials contain 0.01 to 5%, in particular 0.1 to 3%, very preferably 0.2 to 2% by weight of surfactants.
A particularly preferred polymerizable CLC mixture according to the first preferred embodiment comprises
a) 5 to 80%, very preferably 10 to 65% by weight of up to five, preferably one, two or three monoreactive mesogenic compounds having a polar terminal group,
b) 10 to 90%, very preferably 25 to 75% by weight of up to four, preferably one or two, direactive polymerizable mesogenic compounds,
c) 0.1 to 15%, very preferably 0.2 to 9% by weight of up to three, preferably one or two, non-polymerizable chiral dopants,
d) 0.5 to 10%, very preferably 1 to 7% by weight of a polymerization initiator,
e) 0 to 5%, very preferably 0.1 to 2% by weight of one or more surfactants.
The compounds of component a) are preferably selected from the above formulae Va and Vb.
The compounds of component b) are preferably selected from the above formulae VIa and VIb. 
The chiral dopants of component c) are preferably selected from the above formulae VII and VIII.
The surfactants of component e) are preferably selected from the above formulae IX and X.
According to the inventive method, the polymerizable CLC material is coated onto a substrate and aligned into a uniform orientation. It can then be subsequently polymerized to permanently fix the orientation of the cholesteric material.
As a substrate for example a glass or quarz sheet or a plastic film or sheet can be used. In case of polymerizable matetrials, the substrates can be removed after polymerization or not. In case the substrate is not removed from the polymerized film after polymerization, preferably an isotropic substrate is used.
Preferably the substrate is a plastic substrate such as for example a film of polyester such as polyethyleneterephthalate (PET), of polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC), especially preferably a PET film or a TAC film. As a birefringent substrate for example an uniaxially stretched plastic film can be used. For example PET films are commercially available from ICI Corp. under the trade name Melinex.
The CLC material can also be dissolved in a solvent, preferably in an organic solvent. The solution is then coated onto the substrate, for example by spin-coating or other known techniques, and the solvent is evaporated off. In most cases it is suitable to heat the mixture in order to facilitate the evaporation of the solvent.
In addition to the methods described above, planar alignment in the coated layer of the CLC material can further be enhanced by shearing the material, e.g. by means of a doctor blade. It is also possible to apply an alignment layer, for example a layer of rubbed polyimide or sputtered SiOx, on top of the substrate, or alternatively to directly rub the substrate, i.e. without applying an additional alignment layer.
For example rubbing can be achieved by means of a rubbing cloth, such as a velvet cloth, or with a flat bar coated with a rubbing cloth. In a preferred embodiment of the present invention rubbing is achieved by means of a at least one rubbing roller, like e.g. a fast spinning roller that is brushing across the substrate, or by putting the substrate between at least two rollers, wherein in each case at least one of the rollers is optionally covered with a rubbing cloth. In another preferred embodiment of the present invention rubbing is achieved by wrapping the substrate at least partially at a defined angle around a roller that is preferably coated with a rubbing cloth.
Polymerization of the polymerizable CLC material takes place by exposing it to heat or actinic radiation. Actinic radiation means irradiation with light, like UV light, IR light or visible light, irradiation with X-rays or gamma rays or irradiation with high energy particles, such as ions or electrons. Preferably polymerization is carried out by UV irradiation.
As a source for actinic radiation for example a single UV lamp or a set of UV lamps can be used. When using a high lamp power the curing time can be reduced. Another possible source for actinic radiation is a laser, like e.g. a UV laser, an IR laser or a visible laser.
The polymerization is carried out in the presence of an initiator absorbing at the wavelength of the actinic radiation. For example, when polymerizing by means of UV light, a photoinitiator can be used that decomposes under UV irradiation to produce free radicals or ions that start the polymerization reaction.
When curing polymerizable mesogens with acrylate or methacrylate groups, preferably a radical photoinitiator is used, when curing polymerizable mesogens with vinyl and epoxide groups, preferably a cationic photoinitiator is used.
It is also possible to use a polymerization initiator that decomposes when heated to produce free radicals or ions that start the polymerization.
As a photoinitiator for radical polymerization for example the commercially available Irgacure 651, Irgacure 184, Darocure 1173 or Darocure 4205 (all from Ciba Geigy AG) can be used, whereas in case of cationic photopolymerization the commercially available UVI 6974 (Union Carbide) can be used.
The polymerizable CLC mixture preferably comprises 0.01 to 10%, very preferably 0.05 to 5%, in particular 0.1 to 3% of a polymerization initiator. UV photoinitiators are preferred, in particular radicalic UV photoinitiators.
The curing time is dependening, inter alia, on the reactivity of the polymerizable mesogenic material, the thickness of the coated layer, the type of polymerization initiator and the power of the UV lamp. The curing time according to the invention is preferably not longer than 10 minutes, particularly preferably not longer than 5 minutes and very particularly preferably shorter than 2 minutes. For mass production short curing times of 3 minutes or less, very preferably of 1 minute or less, in particular of 30 seconds or less, are preferred.
In addition to polymerization initiators the polymerizable material may also comprise one or more other suitable components such as, for example, catalysts, stabilizers, chain-transfer agents, co-reacting monomers or surface-active compounds. In particular the addition of stabilizers is preferred in order to prevent undesired spontaneous polymerization of the polymerizable material for example during storage.
As stabilizers in principal all compounds can be used that are known to the skilled in the art for this purpose. These compounds are commercially available in a broad variety. Typical examples for stabilizers are 4-ethoxyphenol or butylated hydroxytoluene (BHT).
Other additives, like e.g. chain transfer agents, can also be added to the polymerizable material in order to modify the physical properties of the inventive polymer film. When adding a chain transfer agent, such as monofunctional thiol compounds like e.g. dodecane thiol or multifunctional thiol compounds like e.g. trimethylpropane tri(3-mercaptopropionate), to the polymerizable material, the length of the free polymer chains and/or the length of the polymer chains between two crosslinks in the inventive polymer film can be controlled. When the amount of the chain transfer agent is increased, the polymer chain length in the obtained polymer film is decreasing.
It is also possible, in order to increase crosslinking of the polymers, to add up to 20% of a non mesogenic compound with two or more polymerizable functional groups to the polymerizable material alternatively or in addition to the di- or multifunctional polymerizable mesogenic compounds to increase crosslinking of the polymer.
Typical examples for difunctional non mesogenic monomers are alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typical examples for non mesogenic monomers with more than two polymerizable groups are trimethylpropanetrimethacrylate or pentaerythritoltetraacrylate.
In another preferred embodiment the polymerizable CLC mixture comprises up to 70%, preferably 3 to 50% of a non mesogenic compound with one polymerizable functional group. Typical examples for monofunctional non mesogenic monomers are alkylacrylates or alkylmethacrylates.
It is also possible to add, for example, a quantity of up to 20% by weight of a non polymerizable liquid-crystalline compound to adapt the optical properties of the optical retardation film.
In a preferred embodiment of the invention the polymerization of the polymerizable CLC mixture is carried out under an atmosphere of inert gas, preferably under a nitrogen atmosphere.
The selection of suitable polymerization temperatures depends mainly on the clearing point of the polymerizable material and inter alia on the softening point of the substrate. Preferably the polymerization temperature is at least 30 degrees below the clearing temperature of the polymerizable mesogenic mixture.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.
The entire disclosure of all applications, patents and publications, cited above or below, and of corresponding European application No. 99112830.4, filed Jul. 2, 1999 is hereby incorporated by reference.
The following abbreviations are used to illustrate the liquid crystalline phase behavior of the compounds: K=crystalline; N=nematic; S=smectic; Ch=cholesteric; I=isotropic. The numbers between the symbols indicate the phase transition temperatures in xc2x0 C.
Polymerizable CLC mixture 1 with high twist and reflecting blue light and polymerizable CLC mixture 2 with low twist and reflecting red light were formulated
Compounds (1) and (2) can be prepared according to or in analogy to the met hods described in D. J. Broer et al., Makromol. Chem. 190, 3201-3215 (1989). The preparation of compounds (3) and (4) is described in DE 195, 04, 224. The preparation of the chiral dopant (5) is described in WO 98/00428. Irgacure 369 is a commercially available photoinitiator (from Ciba Geigy). FC 171 is a commercially available non-ionic fluorocarbon surfactant (from 3M Corp.).
Each mixture was dissolved in xylene to give a 50% solution.
Blue mixture 1 was spin-coated onto a glass substrate with a rubbed polyimide alignment layer at a spin speed of 3000 rpm. The cholesteric layer was annealed at 60xc2x0 C. for 30 seconds, and then polymerized by exp osure to UV radiation (wavelength 360 nm, intensity 13 mW/cm1) for 1 minute under a nitrogen atmosphere, to give the one-layer polymer film 1A.
The process was repeated for red mixture 2 to give the one-layer 25 polymer film 1C.
Furthermore, the xylene solution of red mixture 2 was coated and polymerized as described above on top of the polymerized layer of blue mixture 1 to give the two-layer polymer film 1B.
The process was repeated, b ut now by coating and polymerizing blue mixture 1 on top of the polymerized layer of red mixture 2, to give the two-layer polymer film 1D.
The visible light absorption spectrum of each of films 1A-1D was measured (Hitachi U2000 spectrophotometer) using circularly polarized light, the reflection from the cholesteric layer appearing as an absorption peak in the spectrum. The spectra are shown in FIG. 1 (curve A-D). Table 1 shows the wavelength of the center of the reflection peak for films 1A-1D (xcex), the difference of xcex between the peaks of two-layer films and their corresponding one-layer films (xcex94xcex), and the relative change of xcex (|xcex94xcex/xcex|).
Film 1A of blue mixture 1, comprising the chiral compounds (3) and (4) and additionally comprising the high HTP dopant (5), shows a short pitch (i.e. high twist) with selective reflection of blue light. Film 1C of red mixture 2 without the additional dopant (5) shows a long pitch (i.e. low twist) with selective reflection of red light. The two-layer films 1B and 1D show two reflection peaks, each corresponding to one of the polymerized layers of mixture 1 and 2.
The shift xcex94xcex of the reflection wavelength of the two peaks in the two-layer films 1B and 1D compared to the corresponding peak in the single-layer films 1A and 1C is shown in Table 1, together with the relative change of the reflection wavelength |xcex94xcex/xcex|. It can be seen that, in case of film 1B, where blue mixture 1 was coated first, there is a significant shift xcex94xcex of the longer wavelength peak (Peak 2) to shorter wavelengths, compared to the same peak in one-layer film 1C. This can be explained by the non-polymerizable chiral dopant (5) migrating from the first, polymerized layer of mixture 1 into the second, non-polymerized layer of mixture 2 during the an nealing of the second layer. The resulting increase in concentration of chiral material in the second layer shifts its reflection peak to shorter wavelengths.
On the other hand, the shorter wavelength peak (Peak 1) in film 1B is only shifted to a small amount towards longer wavelengths, compared to th e same peak in one-layer film 1A. The shif t of Peak 1 is much smaller than observed for Peak 2, because in the first layer (mixture 1) with the short wavelength peak (Peak 1) the cholesteric structure is already fixed by crosslinking. Here, the shift of peak 1 is mainly due to swelling of the first layer by the unpolymerized material of the second layer, which increases the l ayer thichkess and thus increases the pitch, whereas the loss of chiral dopant (5) migrating out of the first layer layer does not have a significant influence on its helical pitch and reflection wavelength.
In case of film 1D, where mixture 2 was coated first, the shift of both the peaks with long er wavelength (Peak 2) and shorter wavelength (Peak 1) is much smaller compared to film 1B. This is because n ow the first layer (mixture 2) does not contain any non-polymerizable chiral material (compound (5)) that can migrate into the second layer.
Polymerizable CLC mixture 3 with high twist a nd reflecting blue light and polymerizable CLC mixture 4 with low twist and reflecting red light were formulated
Compounds (6) and (7) can be prepared according to or in analogy to the methods described in D. J. Broer et al., Makromol. Chem. 190, 3201-3215 (1989). The preparation of the chiral dopant (8) is described in WO 98/00428. Irgacure 651 is a commercially available photoinitiator (from Ciba Geigy).
Each mixture was dissolved in xylene to give a 50% solution.
Blue mixture 3 was spin-coated onto a glass substrate with a rubbed polyimide alignment layer at a spin speed of 3000 rpm. The cholesteric layer was annealed at 84 xc2x0 C. for 30 seconds, and then polymerized by exposure to UV radiation (wavelength 360 nm, intensity 13 mW/cm2) for 1 minute under a nitrogen atmosphere, to give the one-layer polymer film 2A.
The process was repeated for red mixture 4 to give the one-layer polymer film 2B.
Furthermore, the xylene solution of red mixture 4 was spin-coated as described above onto the polymerized layer of blue mixture 3. The unpolymerized layer of mixture 4 was then annealed at a temperature of 84xc2x0 C. for various periods of time, and subsequently polymerized as described above, to give the two-layer films 2C-2G.
The visible light absorption spectrum of each of films 2A-2G was measured (Hitachi U2000 spectrophotometer) using circularly polarized light, the reflection from the cholesteric layer appearing as an absorption peak in the spectrum. The spectra for films 2B-2G are shown in FIG. 2 (curve B-G). Table 2 shows the wavelength of the center of the reflection peak for films 2A-2G (xcex), the difference of xcex between the peaks of two-layer films 2C-2G and their corresponding one-layer films (xcex94xcex), and the relative change of xcex (|xcex1xcex/xcex|)
It can be seen from FIG. 2 and Table 2 that with increasing annealing time X of the second layer moves to progressively shorter wavelengths. At the same time xcex of the first layer remains substantially unaffected.
The decrease of xcex in the second can be explained by diffusion of the non-polymerizable choral dopant (8) from the first layer into the second layer. On the other hand, X of the first layer does not change remarkably due to its high degree of crosslinking, therefore the loss of chiral dopant (8) migrating out of this layer does not have a significant influence on its helical pitch and reflection wavelength.
Several samples of a one-layer polymer film of polymerizable blue CLC mixture 3 of example 2 were prepared as described in example 2. Each sample was treated with a different solvent to remove unpolymerized material from the film. This resulted in a change of the reflection wavelength of the films as shown in table 3.
From table 3 it can be seen that treatment with solvents a-d results only in a negligible change of the reflection wavelength xcex, whereas treatment with solvents e-i leads to a significant change of xcex. Thus, solvents e-i have a larger penetrating effect on the polymer film than solvents a-d.